1. Field of the Invention
The present invention relates to a Code Division Multiple Access (CDMA) demodulation circuit and a CDMA demodulation method, and more particularly to the CDMA demodulation circuit and the CDMA demodulation method used for mobile communication devices.
2. Description of the Related Art
Conventionally, there has been a drawback in the mobile communication environment that fading occurs by transmitted radio waves being reflected and diffracted so that accurate timing of paths cannot be estimated, which causes degradation in receiving characteristics. Hence, examples of means for solving this drawback have been disclosed in a Japanese Patent Laid-Open No. 11-4213 (hereafter referred to as a document 1), a Japanese Patent Laid-Open No. 11-274982 (hereafter referred to as a document 2), a Japanese Patent Laid-Open No. 10-271034 (hereafter referred to as a document 3), a Japanese Patent Laid-Open No. 11-251962 (hereafter referred to as a document 4), and a Japanese Patent Laid-Open No. 11-261528 (hereafter referred to as a document 5).
The technique disclosed in the document 1 is to determine a cycle for integrating correlation power according to a length of an actual fading cycle. The technique disclosed in the document 2 is to measure a delay profile of a baseband signal while changing a measuring time for measuring the delay profile, and to select a desired number of scattered waves based on the measured delay profile to assign respective reception timing to the corresponding de-spreading circuit. The technique disclosed in the document 3 is to provide delay profile calculating means for applying moving average for a moving average period corresponding to a timing fluctuation in a selected path, allowing path movement to follow a rapid timing fluctuation in the selected path by decreasing the moving average period when the timing fluctuation is big and by increasing it when the fluctuation is small. The technique disclosed in the document 4 is to search a reception path timing by means of data received at a high reception level and to perform correlation and combination according to the reception paths. The technique disclosed in the document 5 comprises a searcher unit that obtains sequences of measurements from a multipath profile, the searcher unit having a predetermined timing resolution that separates adjacent measurements within the sequence. The technique reads the sequences of measurements, derives timing offsets having resolutions higher than the predetermined timing resolution of the searcher unit, and assigns finger processing elements to the best candidate paths using the timing offsets.
Among the above documents, the technique disclosed in the document 1 in particular measures the length of the actual fading cycle to determine the most suitable cycle for that length to integrate the correlation power. In contrast, the present invention measures times at which the level drops due to actual fading and, from these drop times, estimates a time at which the level drops due to the next fading. Then, it controls assignment of path timing to a fingers portion before the level drops. Such a technique for estimating the time at which the level drops due to fading is not disclosed in any of the above referred documents from 2 to 5.